Most of my recent lens acquisitions have been of the used variety, so I probably give them a closer look than I would when buying new.

Things I generally check for on a used lens are marks or scratches on the front and rear glass, fungus on the glass, oil on aperture blades, stickiness of aperture blades, dust inside the lens, mechanical smoothness/holding of focus and zoom rings, and functioning threads on the filter ring.

I then take tripod-mounted, remote release shots of a focus target with each of my camera bodies at every focal length/aperture combination. Here is my obviously home-made focus target of newsprint taped to staggered cassette boxes. ( D2H, 28-70, f/2.8 at 70mm)

My tests are primarily to check for back focus/front focus and general sharpness. I am less concerned with corner softness, but that's just me.
My test is far from scientific, but is enough to satisfy my level of concern and ease my anxiety.

I generally try to shoot my tests at slightly beyond the minimum focus distance for the given lens. I find that if I increase the camera-focus target distance too much, then it's harder for me to judge the focus quality.

No, I've been fortunate and have never had to try and return a lens. My first D70 had a backfocus problem that Nikon cured.

To test for that problem on your Nikon 28-70, you could just tape a full newspaper sheet to a flat surface like a wall. Any softness on one side or the other should be revealed by discrepancies in the clarity of the newsprint.

By staggering the newsprint-covered cassette boxes at different depths, and focusing on a box at a middle depth, you can determine by clarity of the newsprint whether there is back/front focusing. I stagger them by the depth of each cassette box.

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